From what I have read, it is not overly common, but it isn't unheard of. I am far from an expert, and haven't seen it yet. One of the big contributors to it is a lack of proper supplements and/or lighting. The cham develops deficiencies and it affects the tongue. It can also occur from damage when the cham shoots its tongue and wraps it around something, or bites it, etc. I am sorry your vet didn't see it as an option. Maybe there was more going on with the cham that led to his or her reasoning.
Heika